Current in Secondary Cable Shielding Coupling to Secondary Ports in Substation

Electromagnetic transient will induce a relative potential difference between earthing conductors and current in secondary cable shielding in substation. The current can couple to the secondary ports through control cables, which may cause operation failure even damage of secondary equipment. Consequently, we obtained the characteristics of the voltage and current coupled to the secondary ports through the experimental simulation, and analyzed the coupling mechanism. First, we employed established test platforms for simulating switching, short circuit and lightning transients to carry out numeral experiments. The experimental results indicate that the voltage waveforms at both terminal ports have the same profile but the opposite polarity, under the condition of the same component at the terminals for all kinds of transients. Then, only common-mode voltage with no differential-mode voltage exists for the twin core cable. Based on the results, an equivalent circuit of secondary cable was proposed. It is revealed that the current in the cable shielding layer will induce the resistance voltage drop, i.e. the voltage at the secondary port. The results also show that the coupling coefficient decreases with the increase of the frequency, and the coupling impedance, which is independent of frequency, is close to the resistance of the cable shielding layer. All these results will be helpful to further research on the electromagnetic protection of the secondary ports. © 2018, High Voltage Engineering Editorial Department of CEPRI. All right reserved.